The goal of this project is to understand the molecular mechanisms that underlie the pathogenesis of acute promyelocytic leukemia (APL). When PML-RARalpha is expressed in the early myeloid cells of mice, all animals develop myeloproliferative disease, and some develop APL after a long latent period, which suggests that additional mutations are required for APL to progress. Recently, we have learned that APL penetrance is heavily influenced by 1) neutrophil elastase (NE)-induced cleavage of PML-RARalpha and 2) the expression level of PML-RARalpha. To further address the issues raised by these observations, we propose the following Specific Aims: Specific Aim 1: We will create and analyze mice that express an NE-resistant PML-RARalpha cDNA in early myeloid cells. To determine whether the cleavage of PML-RARalpha by NE is relevant for leukemia development, we have created a mutant PML-RARalpha that is minimally cleaved by NE. This cDNA will be targeted to the murine cathepsin G locus using the identical strategy used to make the mCGPR/+ mouse. Mice will be followed for the development of APL, and carefully compared to mCGPR/+ mice expressing wild-type PML-RARalpha. Specific Aim 2: We will create and analyze mice containing a PML-RARalpha cDNA that is conditionally expressed in early myeloid cells under control of the murine PML locus. PML-RARalpha will be targeted to the murine PML 5'UT just downstream from a Lox-stop-Lox cassette. PML-RARalpha will be activated in mice by a cathepsin G-driven Cre transgene. This strategy should limit PML-RARalpha expression to the early myeloid compartment, with expression levels that closely resemble the "physiologic" dose in t(15;17) APL cells. The phenotype of these mice will be carefully compared to our previous models. Specific Aim 3: We will use qenomic approaches to identify genetic events that contribute to APL initiation and progression. High-density BAC arrays will be used in comparative genomic hybridization studies to identify gene copy number changes that may be relevant for APL progression. Affymetrix-based arrays will be used to study the RNA expression profiles of normal and APL promyelocytes. Candidate genes identified by these studies will be resequenced in human AML samples and validated in mouse models.